As the schools break up for summer, the shelves of Britain’s retailers are groaning with “half price” sun protection cream offers, ready for families heading to the beach. Boots’s entire Soltan range is half price. Tesco is offering “two for £12” on branded cream normally costing £7.50 each. Morrisons has Nivea at “better than half price”. But an investigation by Guardian Money casts doubt on the claims made by the retailers – with some of the deals arguably as fake as an orange spray-tan.We asked mySupermarket.co.uk, which compares prices at all the major chains, to track the pricing on some of the major brands and own-brands sold by the retailers over the past year.

What we found mirrors our investigation into wine prices last year: a zig-zag pattern where prices are “established” at what critics say are artificially high levels so they can be cut to “half price” to give shoppers the false impression they are obtaining a bargain.

We also found supermarkets which, even more cynically, raise the price for a short period before the summer begins, then offer two bottles for a reduced price – which is actually higher than before the “promotion”. Shoppers are therefore tricked into buying more, at an inflated price.

The first retailer we checked was Boots, as it is one of the biggest sellers of sun cream in the UK, and is pushing half-price creams as part of a “Get the most out of summer” promotion. We price-checked one of its Soltan Kids’ creams (the Hypoallergenic Suncare Spray SPF 50+,) as they are likely to be a popular purchase once schools are out. Boots says the cream is on a ­half-price offer at £5.25, reduced from £10.50.

But when was it £10.50? Not last summer – then it was £5.49. In the autumn the price went up to £10.99 – and was instantly moved to “buy one, get one free”.

Indeed, the only time where we could find Boots selling it at £10.50 was between January and March this year, when it was also on “bogof”. Since then it’s been at the £5.25 level.

Department for Business rules for traders (pdf) state that “the price used as a basis for comparison should have been your most recent price available for 28 consecutive days or more; and the period for which the new (lower) price will be available should not be so long that the comparison becomes misleading”.

We asked Boots if its sun cream offer was a genuine discount, and why it “establishes” the price at a time when few families buy the product (eg, in November or January, when children are in school), then cuts the price.

Its press office responded: “At Boots UK we are committed to offering our customers great value on sun protection when they need it most. We continually review our prices to ensure that the full range of brands we sell are priced competitively.” When we contested that it hadn’t answered the question about the specific promotion, a spokeswoman said: “I’m sorry, I can’t be any more help.”

At Tesco, our analysis of mySupermarket.co.uk data found an even stranger pattern. It is selling nearly all its Nivea sun creams for £7.50 each or a “two for £12” deal. But the £7.50 basic price is a curious creation.

This time last year, Tesco was selling the cream (we checked Nivea SPF30 sun lotion) for just £5. Then it put the price up to £10 in late September, where it stayed until March this year, when it went to £5 again.

In June, it was increased to £7.50, but instantly went to “two for £12”. In other words, shoppers buying a single bottle now are paying 50% more than in May, and even if they are seduced by the “two for …” offer it’s still £2 extra.

Sainsbury’s is no stranger to this approach, either. We looked at its own-brand Sun Protect. The bottle we price-checked (the 5-star UVA SPF 15 spray) cost £4, or two for £7.50. At the start of the year, it was selling at Sainsbury’s for a standard £3. Someone popping two of the bottles into their trolley to take advantage of the deal is paying 25% more than earlier in the year.

Over at Morrisons, shoppers will be delighted to hear that our price check showed the £5 it charges for a bottle of Nivea sun spray (SPF 15) is the cheapest in the UK (it is £7.50 at Tesco).

Morrisons goes even further, saying on its website that the deal is “better than half price” as it was previously £11.99. But it was only at that price between November 2013 and the end of March – arguably at a time when few are buying – and was £5 before and after.

Asda, which prides itself on its “everyday low prices” has moved the cost of Ambre Solaire (we checked a sun spray SPF 15) from £5 to £10 and then back to £5, although, to its credit, it is not marketing the cream as half price. Boots is calling the same cream “half price” even though it is more expensive, at £6.50.

Beach accessories, but at what price? Photograph: Alamy

We showed our findings to Which?, the consumer organisation, that is running a campaign to help “Make Special Offers Special”. When it checked offers and promotions on 75 items in November 2013 it found numerous examples of misleading multi-buys and dodgy discounts.

Sainsbury’s put a handwash on special offer for 84 days, saying it “was £1.80, now 90p” whereas it had only been at the higher price for seven days. An Asda multi-buy didn’t save customers any money or, in some cases ended up costing more. The supermarket increased the regular price of Muller Light Greek Style Yoghurt (4 x 120g) from £1.50 to £2.18 as it went on a “two for £4” offer, costing shoppers £1 more. Ocado sold a 12-pack of Beck’s Bier as “was £12.19 now £9” for almost a month but it had only sold at the higher price of £12.19 for three days, 18 days before the offer started.

Which? executive director, Richard Lloyd, says: “We want special offers to be special. We’ve found dodgy deals across the aisles, with prices yo-yoing between multi-buys and discounts, so that it is almost impossible to know the actual price. We’re campaigning for simpler, clearer and fairer pricing rules and tougher enforcement action.”

The pricing strategy for sun lotions uncovered by Guardian Money is not illegal, but is it unfair?

A Tesco spokesperson said: “We aim to offer our customers consistently low prices to help them with their everyday shop, as well as offering promotions on certain products. The Nivea Moisturising Sun Lotion has been available on half price and two for £12 offers since March this year, at a time when many of our customers are preparing to go on holiday.”

Morrisons defended its sale, saying: “Regardless of the time of year, all retailers want to be able to cut the price of a product when cyclical demand rises, and shout about it through marketing. This requires the higher price to be established before demand begins to grow. This particular product was available at £11.99 between 4 November 2013 and 31 March 2014.”

None of the retailers disputed the mySupermarket.co.uk data, which has become the industry standard for price checking.

A spokeswoman said: “Sunscreen is a great product to stock up on when there are good offers, as you can store it even until next summer. At mySupermarket online, and through our mobile app, shoppers can find the best offers across nine retailers, including Boots and Superdrug.”

Not all sun cream does what it says on the tin. Photograph: WestEnd61/Rex Features

Which sun creams work? Hint: price is no guide

Some of the best-known sun creams on the market don’t offer the protection­ they claim, according to consumer group Which? It gave three – from Malibu,­ Piz Buin and Hawaiian Tropic – a “don’t buy” rating, writes Miles Brignall

And paying more, Which? found, doesn’t guarantee better results with the more expensive brands tested outperformed by a cheap rival from the likes of Aldi. In May it tested 15 popular creams that claimed to have a sun protection­ factor (SPF) of 30.

The three that failed were found to have a level of less than 25, while the Malibu product­ also fell short of the required UVA standard.

Researchers tested the creams on 10 volunteers, who had the same amount of each product applied to a small area of their back before lying under a lamp to replicate the sun’s rays.

Their skin was checked for redness and compared to the results of a lamp test with no cream.

The costliest of those on test, Piz Buin Ultra Light Dry Touch Sun Fluid SPF30, at £11.30 per 100ml, failed the SPF test. However, Calypso Sun Lotion SPF30, at £1.20 per 100ml is around a 10th of the cost of Piz Buin lotion – passed both tests.

Richard Lloyd, executive director at Which?, says: “With thousands of cases of skin cancer diagnosed every year, it is vital you can trust a sun cream to provide the protection it claims.”

He says he wants to see manufacturers doing much more to make sure their sun creams “live up to the claims on the packaging”. The makers of the three named creams each disputed the Which?’ findings and have insisted their products meet the claimed protection levels.

Since 2011, we've been asking readers to send in pictures of daft deals they've spotted while out on the high street. Help us add to the collection, and name and shame the retailers who can't do their maths